The present invention relates to a joining method, in particular for connecting at least two workpieces, in which during a joining operation the profile of a force exerted by a joining tool is recorded and compared with a reference force profile in order to be able to check the quality of the joined connection produced.
Joining methods of this type are generally known, for example in the form of a self-piercing riveting method, a clinch-joining method or a solid riveting method. In the case of these joining methods, but also of other types of joining methods, the profile of a force exerted by a joining tool is recorded and compared with a previously recorded reference force profile. The reference force profile is stored during the carrying out of an exemplary joining method. It is known in this case to define a so-called envelope curve in relation to this reference force profile. Here, the reference force curve is shifted by a specific absolute value in the positive direction in order to define an upper threshold curve. Correspondingly, the reference force profile is shifted by a specific absolute value into the negative in order to define a lower threshold curve. The two threshold curves form the envelope curve. Subsequent joining operations are recorded with regard to the force profile and plotted in a diagram in common with the envelope curve. It is therefore possible to determine visually for each joining operation whether the force profile of the joining operation carried out has remained within the envelope curve. If this is the case, the quality of the joined connection produced is judged to be good. Otherwise, the quality is judged to be poor and a defect signal or similar is output.
This type of defect monitoring is relatively inflexible, and it is only by visual comparison of a plurality of such diagrams that it is possible to detect a trend that points, for example, to an increase in wear, fatigue phenomena or the like.
Document DE 197 31 222 B4 discloses a method for forming a self-piercing riveted joint in the case of which a force of a punch and of a hold-down is measured as a function of the travel of the punch or of the hold-down, and an actual profile is determined, a signal being triggered when the determined actual profile deviates from a desired profile in at least one prescribed range about a prescribed limit value.
It is the object of the invention to specify an improved joining method against this background.